Positional determination devices which use stepping motors as a source of motive force are disclosed in JP-Tokkai Hei 7-310811, Tokkai Hei 8-178063 and Tokkai Hei 9-317570.
This device is provided with a feed mechanism which is driven by a stepping motor. An engaging nut with a screw shaft compose the feed mechanism, the nut displaces in response to the rotation of the screw shaft. The nut is connected with a control means and its position is controlled by the rotation of the stepping motor. Pulse signals are utilized for controlling rotation of the stepping motor. The number of pulses determines the amount of rotation, that is to say, the displacement.
The displacement of the feed mechanism diverges from its target position if the stepping motor does not correctly rotate corresponding to the number of input pulses. Although this kind of divergence, in other words, irregularity in the stepping motor, does not happen very often, it does happen on occasion. It is necessary to initialize the stepping motor feed mechanism to correct such a divergence.
For example, in Tokkai Hei 8-178063, stoppers are respectively provided on the limiting positions of the displacement range of the nut. When initializing, the stepping motor is rotated, the nut is displaced until it hits either of the stoppers and its terminal position is set as the initializing position.
When initializing, if there is a divergence in the nut position, the nut will collide with the stopper before reaching the number of rotations which the stepping motor has previously calculated. In this case, the force of the collision due to the speed of the stepping motor's rotation causes the stepping motor to rebound or reverse. The stepping motor reverses in the same sequential rotation in the opposite direction by the number of input pulses left over. This phenomenon is particularly likely to occur in stepping motors with a small number of magnetic poles. As a result, the position of the nut when the stepping motor has stopped rotating does not correspond with the position of the stopper. Therefore in this situation the problem has arisen that initializing has not been correctly performed.